Micromachined pressure sensors are widely incorporated into diverse equipment such as medical, laboratory, and industrial equipment and automotive circuitry. Smaller, more accurate pressure sensors are required for a new generation of equipment in the medical, analytical, and industrial fields while the cost of such pressure sensors must remain low in order to achieve advances at competitive prices.
Typical micromachined pressure sensors can be configured by forming a cavity on the back side of a silicon wafer. Silicon gauge pressure sensors created using semiconductor processes can be made smaller in size by bonding two wafers together. New and novel approaches utilize two wafers bonded together to create a diaphragm. In this approach the first wafer has a cavity formed on one side to set the diaphragm size. A second wafer is then bonded to the first over the cavities. The second wafer can either be the required diaphragm thickness or be thinned to the required thickness. In micro-machined pressure sensors that measure gauge or differential pressure, the cavity in the first wafer must remain open during processing or opened at some point during processing to provide the channel to the back-side of the diaphragm, which is necessary to allow for gauge or differential pressure to be measured. Dirt or debris from processing steps can enter the cavity and remain within the cavity, interfering with the diaphragm as it deflects, which causes the pressure sensor to supply inaccurate read-out of the pressure as measured by the pressure sensor.
The interference due to lodging of minute particulates while configuring gauge pressure sensors can be avoided by forming the back side pressure inlet after completion of all front-side processing. The challenge in this final etch step is to ensure the integrity of the thin diaphragm and the cavity walls supporting the thin diaphragm. In other words, once the backside pressure inlet is fully etched, the etching should immediately stop on the thin diaphragm that is now exposed to the etching chemistry.
In one prior art arrangement, for example, a silicon gauge pressure sensor can be configured by forming a cavity in a first wafer and fusion bonding a second wafer over the first wafer in an oxidizing environment. When etching the back side pressure inlet the thin oxide can be used as an etch stop when the etching reaches the diaphragm. The oxide will etch at a slower rate but if the wafer is not removed from the etch quickly the oxide will break down and the diaphragm will be over-etched.
Based on the foregoing, it is believed that a need exists for an electrochemical etch stopping method that overcomes such problems. It is believed that the system and method disclosed herein offers a solution to these problems by utilizing an electrochemical etch stopping method when performing the final etch to provide an electrical signal of when the etching has reached the diaphragm.